"The minute you're prescriptive, people start checking their watch, texting their girlfriend or boyfriend, they're gone.

"I just give the information, I tell the story... I leave it up to people what to do. I have a very strong opinion, I'm an activist, but I'm a storyteller first and foremost."

It's in the genes, after all. Guggenheim's father Charles won four Academy Awards for his own documentary making - "I've only got one. I'm not going to catch him" - but despite such DNA, his son Davis first tried his luck with mainstream Hollywood fare. And...

"... Got thrown out," is how he remembers it happily, talking to me this week in London at the opening of Siemens' Crystal building, the largest self-sustainable building of its kind, a setting to suit Guggenheim's ongoing mission. "I got kicked to the kerb, so I started making documentaries, and now I want to stay at the kerb."

This enforced career move hasn't served him badly after all. In 2006, he put Al Gore in front of the camera to expound on the world's environmental concerns to Oscar-winning effect.

This summer has seen Guggenheim try to impart his knowledge to different film makers around the world for City Stories, a competition launched by Siemens to seek out short films telling the story of urban landscapes and the ongoing challenge to balance population with sustainability.

If it sounds dry, a glance at the 10 finalists' efforts reveals that worthy needn't mean wearing, something Guggenheim was pleased to discover himself...

"I thought to myself, 'If one of them is good, I've done my job. And we were meant to have three, but in the end we had to choose four. You'd think there'd be a favourite from those four, but I just couldn't... they were all great."

Guggenheim wasn't just a token figure for the judging panel. He spent a significant amount of time - either by phone or skype - with each film maker, from Mumbai to Beijing, Istanbul to Rio, so what did he find?

"The consistency was something common to all good story tellers, there's the passion. And with reason. Earth is getting hotter, there's more CO2 in the air, human behaviour must change."

As well as the idealism, Guggenheim was impressed technically...

"For one film, made in Sydney, the director juxtaposed two voices and intercut them with these alternating opinions, and the effect is very powerful.

So will that be turning up in his next film, the subject of which he's currently holding close to his heart? "No doubt. Artists borrow. Geniuses steal."

Spoken like a true recycler.

Learn more about Siemens Crystal here. Watch the City Stories entry from London below...